I have 4, 1-year-old Lifelines connected to a Magnum 2012 inverter/charger. A few days ago, I noticed that it took quite a while for the battery to charge. I talked to a tech agent from Magnum about this, and he said that it could be that one or more of the battery cells has gone bad, but he didn't tell me how to check for this. How do I do this? I'll be testing a 6V AGM battery.